Servlet 3特性：异步Servlet

解异步Servlet之前，让我们试着理解为什么需要它。假设我们有一个Servlet需要很多的时间来处理，类似下面的内容：

LongRunningServlet.java

package com.journaldev.servlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/LongRunningServlet")
public class LongRunningServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doGet(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
        long startTime = System.currentTimeMillis();
        System.out.println("LongRunningServlet Start::Name="
                + Thread.currentThread().getName() + "::ID="
                + Thread.currentThread().getId());

        String time = request.getParameter("time");
        int secs = Integer.valueOf(time);
        // max 10 seconds
        if (secs > 10000)
            secs = 10000;

        longProcessing(secs);

        PrintWriter out = response.getWriter();
        long endTime = System.currentTimeMillis();
        out.write("Processing done for " + secs + " milliseconds!!");
        System.out.println("LongRunningServlet Start::Name="
                + Thread.currentThread().getName() + "::ID="
                + Thread.currentThread().getId() + "::Time Taken="
                + (endTime - startTime) + " ms.");
    }

    private void longProcessing(int secs) {
        // wait for given time before finishing
        try {
            Thread.sleep(secs);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

如果我们的URL是：http://localhost:8080/AsyncServletExample/LongRunningServlet?time=8000

得到响应为“Processing done for 8000 milliseconds! !“。现在，如果你会查看服务器日志，会得到以下记录:

LongRunningServlet Start::Name=http-bio-8080-exec-34::ID=103
LongRunningServlet Start::Name=http-bio-8080-exec-34::ID=103::Time Taken=8002 ms.

所以Servlet线程实际运行超过 8秒，尽管大多数时间用来处理其它Servlet请求或响应。

这可能导致线程饥饿——因为我们的Servlet线程被阻塞，直到所有的处理完成。如果服务器的请求得到了很多过程，它将达到最大Servlet线程限制和进一步的请求会拒绝连接错误。

Servlet 3.0之前，这些长期运行的线程容器特定的解决方案，我们可以产生一个单独的工作线程完成耗时的任务，然后返回响应客户。Servlet线程返回Servlet池后启动工作线程。Tomcat 的 Comet、WebLogic FutureResponseServlet 和 WebSphere Asynchronous Request Dispatcher都是实现异步处理的很好示例。

容器特定解决方案的问题在于，在不改变应用程序代码时不能移动到其他Servlet容器。这就是为什么在Servlet3.0提供标准的方式异步处理Servlet的同时增加异步Servlet支持。

实现异步Servlet

让我们看看步骤来实现异步Servlet，然后我们将提供异步支持Servlet上面的例子：

1. 首先Servlet,我们提供异步支持 Annotation @WebServlet  的属性asyncSupported 值为true。
2. 由于实际实现委托给另一个线程，我们应该有一个线程池实现。我们可以一个通过Executors framework 创建线程池和使用servlet context listener来初始化线程池。
3. 通过ServletRequest.startAsync方法获取AsyncContext的实例。AsyncContext提供方法让ServletRequest和ServletResponse对象引用。它还提供了使用调度方法将请求转发到另一个 dispatch() 方法。
4. 编写一个可运行的实现,我们将进行重处理，然后使用AsyncContext对象发送请求到另一个资源或使用ServletResponse编写响应对象。一旦处理完成，我们通过AsyncContext.complete()方法通知容器异步处理完成。
5. 添加AsyncListener实现AsyncContext对象实现回调方法，我们可以使用它来提供错误响应客户端装进箱的错误或超时，而异步线程处理。在这里我们也可以做一些清理工作。

一旦我们将完成我们的项目对于异步Servlet示例，项目结构看起来会像下面的图片：

在监听中初始化线程池

package com.journaldev.servlet.async;

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

@WebListener
public class AppContextListener implements ServletContextListener {

    public void contextInitialized(ServletContextEvent servletContextEvent) {

        // create the thread pool
        ThreadPoolExecutor executor = new ThreadPoolExecutor(100, 200, 50000L,
                TimeUnit.MILLISECONDS, new ArrayBlockingQueue<Runnable>(100));
        servletContextEvent.getServletContext().setAttribute("executor",
                executor);

    }

    public void contextDestroyed(ServletContextEvent servletContextEvent) {
        ThreadPoolExecutor executor = (ThreadPoolExecutor) servletContextEvent
                .getServletContext().getAttribute("executor");
        executor.shutdown();
    }

}

实现很直接，如果你不熟悉ThreadPoolExecutor 框架请读线程池的ThreadPoolExecutor 。关于listeners 的更多细节，请阅读教程Servlet Listener。

工作线程实现

package com.journaldev.servlet.async;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.AsyncContext;

public class AsyncRequestProcessor implements Runnable {

    private AsyncContext asyncContext;
    private int secs;

    public AsyncRequestProcessor() {
    }

    public AsyncRequestProcessor(AsyncContext asyncCtx, int secs) {
        this.asyncContext = asyncCtx;
        this.secs = secs;
    }

    @Override
    public void run() {
        System.out.println("Async Supported? "
                + asyncContext.getRequest().isAsyncSupported());
        longProcessing(secs);
        try {
            PrintWriter out = asyncContext.getResponse().getWriter();
            out.write("Processing done for " + secs + " milliseconds!!");
        } catch (IOException e) {
            e.printStackTrace();
        }
        //complete the processing
        asyncContext.complete();
    }

    private void longProcessing(int secs) {
        // wait for given time before finishing
        try {
            Thread.sleep(secs);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

注意：在请求和响应时使用AsyncContext对象，然后在完成时调用 asyncContext.complete() 方法。

AsyncListener 实现

package com.journaldev.servlet.async;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.AsyncEvent;
import javax.servlet.AsyncListener;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebListener;

@WebListener
public class AppAsyncListener implements AsyncListener {

    @Override
    public void onComplete(AsyncEvent asyncEvent) throws IOException {
        System.out.println("AppAsyncListener onComplete");
        // we can do resource cleanup activity here
    }

    @Override
    public void onError(AsyncEvent asyncEvent) throws IOException {
        System.out.println("AppAsyncListener onError");
        //we can return error response to client
    }

    @Override
    public void onStartAsync(AsyncEvent asyncEvent) throws IOException {
        System.out.println("AppAsyncListener onStartAsync");
        //we can log the event here
    }

    @Override
    public void onTimeout(AsyncEvent asyncEvent) throws IOException {
        System.out.println("AppAsyncListener onTimeout");
        //we can send appropriate response to client
        ServletResponse response = asyncEvent.getAsyncContext().getResponse();
        PrintWriter out = response.getWriter();
        out.write("TimeOut Error in Processing");
    }

}

通知的实现在 Timeout()方法，通过它发送超时响应给客户端。

Async Servlet 实现

这是我们的异步Servlet实现，注意使用AsyncContext和ThreadPoolExecutor进行处理。

package com.journaldev.servlet.async;

import java.io.IOException;
import java.util.concurrent.ThreadPoolExecutor;

import javax.servlet.AsyncContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet(urlPatterns = "/AsyncLongRunningServlet", asyncSupported = true)
public class AsyncLongRunningServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doGet(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
        long startTime = System.currentTimeMillis();
        System.out.println("AsyncLongRunningServlet Start::Name="
                + Thread.currentThread().getName() + "::ID="
                + Thread.currentThread().getId());

        request.setAttribute("org.apache.catalina.ASYNC_SUPPORTED", true);

        String time = request.getParameter("time");
        int secs = Integer.valueOf(time);
        // max 10 seconds
        if (secs > 10000)
            secs = 10000;

        AsyncContext asyncCtx = request.startAsync();
        asyncCtx.addListener(new AppAsyncListener());
        asyncCtx.setTimeout(9000);

        ThreadPoolExecutor executor = (ThreadPoolExecutor) request
                .getServletContext().getAttribute("executor");

        executor.execute(new AsyncRequestProcessor(asyncCtx, secs));
        long endTime = System.currentTimeMillis();
        System.out.println("AsyncLongRunningServlet End::Name="
                + Thread.currentThread().getName() + "::ID="
                + Thread.currentThread().getId() + "::Time Taken="
                + (endTime - startTime) + " ms.");
    }

}

Run Async Servlet

现在，当我们将上面运行servlet URL：

http://localhost:8080/AsyncServletExample/AsyncLongRunningServlet?time=8000

得到响应和日志：

AsyncLongRunningServlet Start::Name=http-bio-8080-exec-50::ID=124
AsyncLongRunningServlet End::Name=http-bio-8080-exec-50::ID=124::Time Taken=1 ms.
Async Supported? true
AppAsyncListener onComplete

如果运行时设置time=9999，在客户端超时以后会得到响应超时错误处理和日志：

AsyncLongRunningServlet Start::Name=http-bio-8080-exec-44::ID=117
AsyncLongRunningServlet End::Name=http-bio-8080-exec-44::ID=117::Time Taken=1 ms.
Async Supported? true
AppAsyncListener onTimeout
AppAsyncListener onError
AppAsyncListener onComplete
Exception in thread "pool-5-thread-6" java.lang.IllegalStateException: The request associated with the AsyncContext has already completed processing.
    at org.apache.catalina.core.AsyncContextImpl.check(AsyncContextImpl.java:439)
    at org.apache.catalina.core.AsyncContextImpl.getResponse(AsyncContextImpl.java:197)
    at com.journaldev.servlet.async.AsyncRequestProcessor.run(AsyncRequestProcessor.java:27)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
    at java.lang.Thread.run(Thread.java:680)

注意：Servlet线程执行完，很快就和所有主要的处理工作是发生在其他线程。

这是所有异步Servlet内容，希望你喜欢它。下载 AsyncServletExample 工程

本文由 ImportNew - 彭秦进 翻译自 journaldev